In plasma process for manufacturing a semiconductor device, a liquid crystal display or the like, a high frequency in a MHz band or microwave in a GHz band is used to discharge a process gas in a vacuum process chamber. A microwave discharge type plasma processing apparatus is advantageous in that a plasma having a high density and a low electron temperature can be generated at a low pressure and also in that a configuration of the plasma processing apparatus can be simplified because a magnetic field is not required and a large-diameter plasma can be effectively generated in a wide pressure range by employing a flat plate type microwave introducing window structure.
Generally, in the microwave discharge type plasma processing apparatus, a microwave generator for supplying microwave into a process chamber is provided at a location separated from the process chamber, and an antenna having microwave radiation slots is provided above the microwave introducing window (dielectric window) of the process chamber. Further, the microwave is transmitted from the microwave generator to the antenna through a microwave transmission line. This microwave transmission line is typically configured to transmit the microwave outputted from the microwave generator to a mode transducer in a TE mode or a TM mode through a waveguide and then transmit the microwave that has been converted to a TEM mode by the mode transducer to the antenna through a coaxial waveguide.
In the microwave discharge type plasma processing apparatus, in order to effectively supply a power of the microwave to a load (mainly the plasma), it is required to match an impedance of a load side with an impedance of a microwave generator on the microwave transmission line. Generally, an automatic matcher is attached to the waveguide. Conventionally, a stub tuner, an E tuner, and an EH tuner are well known as the automatic matcher.
In the stub tuner, a plurality of stub rods are arranged at a regular interval in a travelling direction of the microwave and a probe type sensor is disposed at a position closer to the microwave generator than the stub rods to detect a standing wave (travelling wave and reflection wave) of the microwave in the waveguide. The stub rods are movably inserted into a rectangular waveguide from one side surface thereof. A controller of the stub tuner variably controls insertion lengths of the stub rods by controlling a driving of a motor while using an output signal of the sensor as a feedback signal. Accordingly, a characteristic impedance of the waveguide is controlled and, further, the impedance of the load side is matched with the impedance of the microwave generator.
In the E tuner, a plurality of branch waveguides connected to one side surface (E-plane) of a rectangular waveguide are arranged at a regular interval in the traveling direction of the microwave, and a movable short-circuit body referred to as a movable short-circuit plate or a short plunger or the like is provided in each branch waveguide so as to be movable in an axial direction of the waveguide. A controller of the E tuner controls the characteristic impedance of the waveguide by variably controlling the position of the movable short-circuit body in the branch waveguide by controlling a driving of the motor based on an output signal of the probe type sensor described above, and further matches the impedance of the load side with the impedance of the microwave generator.
In the EH tuner, two branch waveguides are connected to an E-plane and an H-plane of the rectangular waveguide and a movable short-circuit body is provided in each branch waveguide so as to be movable in an axial direction of the waveguide, as in the case of the E tuner. A controller of the EH tuner controls the characteristic impedance of the waveguide by variably controlling the position of the movable short-circuit body in the branch waveguide by controlling a driving of the motor based on an output signal of the probe type sensor described above, and further matches the impedance of the load side with the impedance of the microwave generator side (see, e.g., Japanese Patent Application Publication Nos. H2-249301 and 2001-251107).
In the microwave discharge type plasma processing apparatus, when the microwave transmitted from the microwave generator to the antenna through the microwave transmission line is radiated to a plasma generating space in the process chamber through the dielectric window, a gas near an electric field (microwave electric field) of the surface wave transmitted in a radial direction along an inner surface of the dielectric window is discharged. As a result, a plasma having a high density and a low electron temperature is generated.
In the surface wave plasma, a wave number of a standing wave generated on a boundary surface of the dielectric window has a discontinuous value determined by a boundary condition, so that the electron density of the plasma has a discontinuous value or mode. In accordance with another viewpoint, the electron density of the plasma is not definitively determined with respect to an incident power of the microwave and may have a plurality of values or modes. These modes include a stable mode with a small amount of reflection and an unstable mode with a large amount of reflection. Therefore, even if the incident power is increased or decreased, the electron density of the plasma is not necessarily changed in direct proportion to the increase/decrease of the incident power and rather tends to circulate a plurality of modes to repeatedly occur a mode jump while forming a hysteresis curve. If such a mode jump occurs repeatedly, a reflection wave power is generated and the electron density is abruptly changed. As a consequence, there occurs a phenomenon a so-called hunting that the plasma emission becomes intense or weak repeatedly.
In the conventional microwave plasma processing apparatus, the automatic matcher attached to the microwave transmission line (waveguide) does not have a function of effectively preventing the hunting of the plasma emission. Therefore, it is required to prevent stability, reproducibility, and quality of the plasma process from being deteriorated by the hunting.